The present invention relates to a reel for a magnetic tape cassette.
Heretofore, a feeding reel or a winding reel used for a video tape cassette etc. comprises a cylindrical shaft (2) for connecting with a rotary driving shaft and a magnetic tape winding cylindrical core (3) whose diameter is larger than that of the cylindrical shaft (2) which are respectively mounted in a coaxial state on a disk reel hub (1) and reinforcing ribs (4) which are formed in the radial direction between the shaft (2) and the magnetic tape winding core (3).
The reinforcing ribs (4) are required for increasing a mechanical strength of the tape winding core (3) and improving a molten fluidity of a synthetic resin in the molding of the synthetic resin to form the reel. However, the reinforcing ribs (4) cause certain heat shrinkages just after a demolding in the synthetic resin molding operation to deteriorate disadvantageously the roundness degree of the magnetic tape winding core (3) connected with the reinforcing ribs (4). Such disadvantage is further remarkable depending upon an increase of an inner diameter of the magnetic tape winding core (3).
FIG. 3 shows the result of the measurement of roundness degree of the conventional reel for the video tape cassette shown in FIGS. 1 and 2.
In FIG. 3, heat shrinkages (10 .mu.m for one graduation) of the magnetic tape winding core (3) are plotted in its radial direction.
It is understood from FIG. 3, the radius of the magnetic tape winding core (3) is smaller at the positions mounting the reinforcing ribs (4); and the radius is larger at the positions between the reinforcing ribs (4) whereby the reel is deformed in a flowery form as a whole.